1. Field of the Invention
The present invention relates to current mirror circuits, and in particular, to metal oxide semiconductor field effect transistor (MOSFET) current mirror circuits with cascode outputs.
2. Description of the Related Art
Current mirror circuits, in general, are well known in the art and are used in many applications. As is well known, in a conventional current mirror circuit, an input current source drives one of a pair of transistors interconnected in such a manner that such input current is substantially replicated, or mirrored, at the output of the second transistor. Generally, one important factor in designing such a current mirror circuit is matching the input and output currents according to the desired proportion or ratio.
Current mirror circuits found in present day integrated circuits tend to be implemented using MOSFETs. As integrated circuits have become increasingly dense, in terms of transistor count versus die size, channel lengths of the MOSFETs have also become shorter. Such decreased channel lengths result in decreased output impedances for current mirror circuits. Accordingly, it has become increasingly necessary to provide cascode output circuits to increase the output impedances.
Cascode output stages often exhibit limited voltage ranges in terms of possible biasing voltage for the cascode output stage, as well as possible supply voltages. For example, in complementary MOSFET applications requiring a high DC voltage gain, as noted above, a cascode output stage is often implemented. Typically, the bias voltage on the gate of the cascode transistor is a fixed voltage. Similarly, when a differential transistor pair is used, the voltage on the gates of each transistor is also typically a fixed DC common mode voltage. Such fixed voltages limit the drain voltage of the driver transistor for the cascode device.